Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.

In a saucepan, combine water, butter, and salt. Bring to a boil over medium heat.

Remove from heat and add flour all at once, stirring vigorously until a smooth dough forms.

Transfer the dough to a mixing bowl and beat in the eggs one at a time until incorporated.

Pipe or spoon the dough onto the prepared baking sheet in 4-inch long strips.

Bake for 25-30 minutes, or until golden brown and puffed up. Let cool completely.

In a saucepan, whisk together milk, sugar, cornstarch, and salt. Bring to a simmer over medium heat.

In a separate bowl, whisk egg yolks. Temper the yolks with a little of the warm milk mixture.

Return the yolk mixture to the saucepan and cook, whisking constantly, until thickened. Remove from heat and stir in butter and vanilla extract.

Transfer the pastry cream to a bowl, cover with plastic wrap (pressing directly onto the surface), and chill for at least 2 hours.

Place chocolate in a heatproof bowl. Heat the heavy cream in a saucepan until it simmers.

Pour the hot cream over the chocolate and let stand for 1 minute. Stir until smooth.

Let the ganache cool to room temperature.

Cut the cooled choux pastry in half horizontally.

Fill the bottom halves with pastry cream.

Top with the ganache and place the top halves of the choux pastry on top.

Serve immediately or chill until ready to serve.